My mare was like this when I first got her - way smarter and more knowledgeable than I was. She was being shown by her young amateur in the 1.25 m, but had done some bigger classes with a pro. When I got her she rooted so bad she’d literally pull me out of the saddle onto her neck, you could not touch her with your leg - literally rode with my lower leg off her side with visible space between my leg and her side. She had been ridden pretty heavy handed and legged and bullied around, she had a right to be angry and not want to be touched. It honestly took 3 years for me to learn to ride her and for her to finally trust me with her mouth and start having some say in things. Once I learned to ride her I figured out she could do 1 tempi changes, canter pirouettes, transitions from a full gallop to a 10 ft collected stride, lead changes on a circle, the best extended and collected trot I’ve ever sat, etc. with just my thoughts. I mean she was just absolutely amazing to ride both on the flat and jumping - I had flat rides where I’d get off with goosebumps. We bought her as a 1.2 m jumper and she took me to 1.45 m.
So anyway, what worked with her was time and patience and learning to be very still, soft, and correct in my body, put her in softer bits, and earn her trust and respect. It was tough in the beginning for sure, amd lots of people “bit up” horses like this but in a well schooled, sensitive horse, the heaviness is how they are telling us they are not happy and comfortable - this can be in their bodies but also in the equipment being used and how we are riding. We are often too loud for horses like this. I’d recommend a slightly thinner rubber or plastic, consider one with a double break or play around with a thinner mullen; the myler may be a good option, or the leather bit. Ride with your forearms as soft as possible and just repeat “melt like butter” in your head (or a similar mantra). It’s hard to be at the same time very still in your body and soft, without tension, but with time you get it. As far as exercises a lot of transitions can make these types hotter, absolutely do not do any lead changes for a while. Lots of circles and lateral work, mostly in the walk or trot. Also mix in loose rein rides where you don’t touch her, even if you have to stay in the walk at first. But also just spending a lot of time with her on the ground - grooming, grazing. Do a little natural horsemanship. If you have access hand walk her on trails. Once you get this mare on your side, she’ll do anything for you and teach you so much. Enjoy!